UCL and LCL, or upper and lower control limits (Statistical Process Control (SPC), easy)

87. The _________ is a quality control chart used to control the number of defects per unit of output.

C-chart (Statistical Process Control (SPC), moderate)

88. The term _________ is used to describe how well a process makes units within design specifications (or tolerances).

Process capability (Process capability, moderate)

89. A C_pk index greater than _________ is a capable process, one that generates fewer than 2.7 defects per 1000 at the ± level.

Unity, or 1 (Process capability, moderate)

90. ___________ is a method of measuring samples of lots or batches of product against predetermined standards.

Acceptance sampling (Acceptance sampling, moderate)

91. A(n) _____________ is a graph that describes how well an acceptance plan discriminates between good and bad lots.

OC or operating characteristics curve (Acceptance sampling, moderate)

92. The __________ is the poorest level of quality that we are willing to accept.

AQL or acceptable quality level (Acceptance sampling, moderate)

93. The ____________ is the percent defective in an average lot of goods inspected through acceptance sampling.

AOQ or average outgoing quality (Acceptance sampling, moderate)

SHORT ANSWER

94. What is the basic objective of a process control system?

It is to provide a statistical signal when assignable causes of variation are present. (Statistical Process Control (SPC), moderate)

95. Briefly explain what the Central Limit Theorem has to do with control charts.

The CLT underlies the distribution of sample means and the standard deviation of sample means. It leads to the usability of the normal distribution in control charts. (Statistical Process Control (SPC), moderate)

96. What are the three possible results (or findings) from the use of control charts?

The results of a control chart can indicate (a) in control and capable, (b) in control but not capable, and (c) out of control. (Statistical Process Control (SPC), moderate)

97. Why do range charts exist? Aren't x-bar charts enough?

Range charts and mean charts perform different functions. The mean chart is used to detect changes in the average of a process. But that average might stay the same while output is getting more scattered. The purpose of the range chart is to detect changes in the dispersion of a process. (Statistical Process Control (SPC), moderate)

98. Examine the Statistical Process Control outputs below. Answer the following questions.

a. What is the sample size?

b. What is the number of samples?

c. What is the mean of sample 8; what is the range of sample 10?

d.. Is this process in control? Explain--a simple Yes or No is insufficient.

e. What additional steps should the quality assurance team take?

	Mean	Range
UCL (Upper control limit)	12.6856	1.0193
CL (Center line)	12.36	0.67
LCL (Lower control limit)	12.0344
Sample Number	Item1	Item2	Item3	Item4	Mean	Range
Sample 1	12.2	12.6		12.1	12.225	0.6
Sample 2	11.9	12.5	12.4	12.7	12.375	0.8
Sample 3		12.2	12.9	13.1	12.55	1.1
Sample 4	12.5	12.5	12.4	12.8	12.55	0.4
Sample 5	12.2	12.8	12.7		12.425	0.8
Sample 6	12.1	12.5	11.8	12.3	12.175	0.7
Sample 7	12.3	12.4	12.8	12.4	12.475	0.5
Sample 8		12.1	12.4	12.2	12.175	0.4
Sample 9	12.1	12.8	12.4	11.9	12.3	0.9
Sample 10	12.6	12.4	12.1	12.3	12.35	0.5

The sample size is 4; ten samples were taken. The mean of sample 8 is 12.175; the range of sample 10 is 0.5. This chart is built on 2-sigma limits, so the probability of a false signal is about 4.5%. The process is not in control—while all means are within limits, the range for sample 3 is too large. Investigate for assignable cause and eliminate that cause.

(Statistical Process Control (SPC), difficult) {AACSB: Analytic Skills}

99. Can "in control" and "capable" be shown on the same chart?

Only indirectly. The chart illustrating control plots the averages of small samples, while "Capability" is based on the dimensions of individual units. Figure S6.2 suggests that an overly wide range for sample means implies an overly large range for individual values as well. (Statistical Process Control (SPC), moderate)

100. What is the difference between natural and assignable causes of variation?

Natural variations are those variations that are inherent in the process and for which there is no identifiable cause. These variations fall in a natural pattern. Assignable causes are variations beyond those that can be expected to occur because of natural variation. These variations can be traced to a specific cause. (Statistical Process Control (SPC), moderate)

101. Why are and R-charts usually used hand in hand?

The ultimate goal of the and R -charts is to ascertain, by a sampling procedure, that the relevant parameter is kept within specific upper and lower bounds. The X -bar chart alone tells us only that the average or variable values are within the appropriate limits. The combination of and the R -charts allows one to determine that both the average and the deviations are within the limits. (Statistical Process Control (SPC), moderate)

102. What does it mean for a process to be "capable"?

Process capability implies that the natural variation of the process must be small enough to produce products that meet the specifications or tolerances required. (Process capability, moderate)

103. What is the difference between the process capability ratio C_p and the process capability index C_pk?

The C_p ratio does not consider how well the process average is centered on the target value. However, C_pk does consider how well the process is centered. (Process capability, moderate)

104. A process is operating in such a manner that the mean of the process is exactly on the lower specification limit. What must be true about the two measures of capability for this process?

The C_p ratio does not consider how well the process average is centered on the target value; its value is unaffected by the setting for the process mean. However, C_pk does consider how well the process is centered; with x-bar on the LSL, the formula guarantees a C_pk of zero. (Process capability, moderate)

105. What is acceptance sampling?

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